This invention relates to a measuring device and more particularly to an improved scanning unit for use with a digital length measuring device.
Digital length measuring devices are in widespread use today. One form of these measuring devices is described in German Publication Copy DT-OS 25 05 586, and includes a scanning unit which is connected to the object being measured. This scanning unit moves along an indexed scale and includes apparatus for measuring the position of the scanning unit along the scale. Movement of the object causes the scanning unit to move along the scale, and the position of the object being measured can be determined by measuring the position of the scanning unit. An auxilliary guide means is provided to position the scanning unit in two dimensions perpendicular to the direction of measurement. The scanning unit itself includes a plurality of guide bearings which bear on guide surfaces defined by the guide means.
One scanning unit known to the prior art is described in West German Gebrauchsmuster GM 76 20 153. This scanning unit includes three sets of guide bearings which serve to position the scanning unit with respect to three guide surfaces defined by the auxilliary guide means. The first set of guide bearings includes three bearings arranged in a triangle which bear on the first guide surface. The second set of guide bearings includes two bearings arranged in a line which bear on the second guide surface, transverse to the first guide surface. The third set of guide bearings includes a single bearing which is resiliently positioned on a third guide surface placed obliquely opposite the first and second guide planes. The resilient contact between this single bearing and the third guide surface provides a positioning force which urges the first and second sets of bearings against the first and second guide surfaces, respectively.
Scanning units such as those described above exhibit certain disadvantages. In many applications these scanning units introduce measurement errors when irregularities in one or more of the three guide surfaces are encountered. In particular, it is often desirable to construct the auxiliary guide means from several separate lengths of a housing profile which are joined together. The joints between adjacent lengths of housing profiles often introduce irregularities in one or more of the guide surfaces which may introduce measurement errors when a scanning unit of the prior art traverses these joints. Such errors may represent a significant drawback in certain high precision measuring devices.
The present invention is directed to an improved scanning unit which is capable of traversing joints in the guide means with reduced susceptibility to measurement error.